Date of the Glacial and the Upper Miocene Period. 379 



sea is being affected by the transference of sediment from the 

 land to the sea. Such a result, however, is very doubtful ; for it 

 is quite possible that it may be more than neutralized by up- 

 heavals of the land. 



The amount of sediment carried down into the Gulf of Mexico 

 by the Mississippi River has been estimated with the greatest 

 accuracy by Messrs. Humphreys and Abbot. It is found that 

 the average amount of sediment held in suspension in the 

 waters of the Mississippi is about y^,- of the weight of the 

 water, or ^jj^ by bulk. The annual discharge of the river is 

 19,500,000,000,000 cubic feet of water. The quantity of 

 sediment carried down into the Gulf of Mexico amounts to 

 6,724,000,000 cubic feet. But besides that which is held in 

 suspension, the river pushes down into the sea about 750,000,000 

 cubic feet of earthy matter, making in all a total of 7,474,000,000 

 cubic feet transferred from the land to the sea annually. Where 

 does this enormous mass of material come from ? Unquestion- 

 ably it comes from the ground drained by the Mississippi. The 

 area drained by the river is 1,244,000 square miles. Now 

 7,474,000,000 cubic feet removed off 1,224,000 square miles of 

 surface is equal to 45 * r6 of a foot off that surface per annum, or 

 one foot in 4566 years. The specific gravity of the sediment is 

 taken at 1*9, that of rock is about 2*5 ; consequently the amount 

 removed is equal to one foot of rock in about 6000 years. The 

 average height of the North- American continent above the sea- 

 level, according to Humboldt, is 748 feet ; consequently, at the 

 present rate of denudation, the whole area of drainage will be 

 brought down to the sea-level in less than 4,500,000 years* if 

 no elevation of the land takes place. 



The rate of denudation of the area drained by the river 

 Ganges is much greater than this. The annual discbarge of 

 that river is 6,523,000,000,000 cubic feet of water. The sediment 

 held in suspension is equal 5x0 by weight; area of drainage 

 432,480 square miles. This gives 1 foot of rock in 2358 years 

 as the amount removed. 



Rough estimates have been made of the amount of sediment 

 carried down by some eight or ten European rivers ; and although 

 those estimates cannot be depended upon as being anything like 

 perfectly accurate, still they show (what there is very little reason 

 to doubt) that it is extremely probable that the European conti- 

 nent is being denuded about as rapidly as the American. By 

 means of subaerial agencies continents are cut up into islands, 



* My former estimate was incorrect. It was derived from imperfect data 

 obtained previously to the observations of Humphreys and Abbot. And, 

 besides, I omitted to take into account the difference between the specific 

 gravity of sediment and rock. 



